1. Field of the Invention
This invention relates generally to optical fibers, and more particularly to optical fibers for high power laser transmission, such as that for laser drilling and cutting.
2. Description of the Related Art
Many fiber geometries are well known in the art. Exemplary of such prior art optical fiber geometries are step index, graded index, single and multi-mode, birefringence and polarization preserving fibers, and fibers whose cores are doped for generating fiber/laser amplifiers for long distance data transmission. These are fibers made from various glasses, quartz, fused silica, metals, and plastics which are manufactured as straight or tapered fibers. Likewise, fiber geometries such as fiber bundles are used for various imaging systems, are not suitable for high power laser transmission. This is because these fiber bundles burn when impinged with a high power laser beam.
With respect to step index and graded index fibers, step index multi-mode fiber used in high power laser beam transmission suffers from what is known in the art as modal dispersion. This means that each individual mode does not travel at the same longitudinal speed since their length varies with their respective total internal reflection angles within the fiber. To avoid this limitation, designers developed graded index fibers during the early 1980's. This design allows the speed of light to be a function of the radius of the fiber core thus making each individual mode travel at the same longitudinal distance even through their path lengths are different. Light travels faster in lower index of refraction media. Therefore, the further the beam is from the fiber's center axis, the greater its speed. Instead of the laser beam being reflected as in step index fiber, the beam is now bent or continuously refracted in an almost sinusoidal pattern, thus the term "self focusing fiber". The major disadvantage of using graded index fiber for high power laser transmission is that because of the self focusing effect within the fiber, the laser beam is focused within the fiber core generating high energy densities. These high energy densities can cause a fiber core breakdown thus the fiber may self-destruct at average laser powers exceeding approximately 50 watts. Therefore, an advantageous optical fiber, then, would be presented if such amounts of modal dispersion were reduced.
It is apparent from the above that there exists a need in the art for a fiber optic which is capable of handling high power laser transmissions, and which at least equals the safety characteristics of known fiber optics, but which at the same time is capable of substantially reducing modal dispersion of the laser transmission. It is a purpose of this invention to fill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.